Polymer forming fabrics are becoming increasingly more complex to suit the changing demands of the paper-making industry. One major development in the paper-making industry, which has necessitated improvement in the structure of existing forming fabrics, is the increased use of recycled paper fibers. As more and more recycled pulp fibers are introduced into the pulp slurry, the shorter recycled fibers along with the associated pulp contaminants have a deleterious effect on the drainage, cleaning, and wear characteristics of the forming fabric.
While having a fine mesh on the top surface, the forming fabric must maintain a high degree of porosity to afford extraction of large quantities of water from the pulp. Forming fabrics with complex weaves have very small filament interstices which easily become blocked with contaminants during the useful life of the fabric. The contaminants which become embedded in the fabric also promote localized wear on the internal fabric binder.
For example, conventional "triple-layer" fabrics typically have a separate system of mono-filament binding yarns interweaving with and connecting the independent top and bottom plies. The top and bottom plies of the fabric have different moduli of elasticity. As the fabric is trained around the guide rollers at the forming end of the paper-making machine, flexing of the two plies generates stresses and strains which permit a degree of relative longitudinal displacement between the top and bottom plies. The relative displacement causes internal localized wear on the binder and prematurely wears or "saws" the binder before the useful wear life of the fabric's bottom ply is fully utilized. Internal binder wear is greatest at the contact point between the larger bottom warp and the binder. As a result, fabric irregularities and delamination of the two independent top and bottom layers develop which adversely affect the paper web formed on the fabric.
Due to the complexity of their weaves and the presence of the large bottom warp, conventional "triple-layer" fabrics have a high caliper with a large amount of void space within the structure. The fabric retains a significant amount of water in the voids after the belt has travelled past the dewatering elements to the exit end of the forming section of the machine. The drier pulp at the exit end of the forming section then has a tendency to reabsorb the water entrained in the body of the fabric. High caliper also adversely affects the flexibility of the fabric in the machine direction. Flexibility in the machine direction permits "table activities", i.e. agitating the pulp as the belt travels on the forming table to facilitate dispersion of the wood fibers more uniformly throughout the layer of pulp on the fabric, thereby enhancing the uniformity in paper formation on the machine.